This invention relates to liquid phase epitaxy, and, more particularly, to problems of melt-carry-over when using boat-slider apparatus.
Liquid phase epitaxy (LPE) using boat-slider apparatus is a common technique for epitaxially growing layers of Group III-V compound semiconductors such as GaAs-AlGaAs and InP-InGaAsP on single crystal substrates such as GaAs and InP, respectively. Frequently, multiple layers forming heterostructures are grown as part of the process of fabricating numerous devices: lasers, LEDs, photodiodes, and FETs, for example.
In this LPE technique, the boat, made of graphite, has a plurality of wells for carrying molten source solutions (melts), and the slider, also made of graphite, has at least one recess for carrying the single crystal substrate. (The slider has additional recesses for carrying saturation seeds, but discussion of them is not necessary here.) The boat and slider are positioned in a furnace to heat the melts and substrate to a temperature suitable for epitaxial growth. A controlled cooling program is instituted, and then the substrate is sequentially slid under each of the wells for a time period sufficient to effect growth thereon of epitaxial layers of the desired thickness and composition.
One problem with this LPE technique is poor wipe-off; that is, droplets from the melt of one well remain on the epitaxial layer grown from that melt after the slider has been moved to place the substrate/epilayer under the next well/melt. This melt-carry-over problem manifests itself in several ways. The layer surface is rough because the melt droplets start to dissolve the underlying layer/substrate, resulting in dissolution pits. If a layer is grown on a dissolution pit containing melt material, holes are produced in the epilayer because the melt droplets are undersaturated. Furthermore, since melt material can be trapped in these holes during wipe-off, once dissolution pits have formed, holes extending through all of the epilayers of a device can result. As a consequence, devices such as LEDs exhibit increased infant mortality and electrical instability.